Portable horizontal space heaters, i.e. those which have a relatively low profile, are shown in U.S. Pat. No. 3,175,550, issued to R. S. Knapp on Mar. 30, 1965, U.S. Pat. No. 3,059,090, granted to R. S. Waters on Oct. 16, 1962, and U.S. Pat. No. 3,610,882, issued to W. A. Omohundra on Oct. 5, 1971. In addition, there are several different models of portable horizontal space heaters currently being marketed in the United States.
A typical portable space heater on the market today has a user-adjustable thermostat used to detect the ambient temperature or, more precisely, the temperature of a portion of the heater that bears a reasonably constant relation to the ambient temperature. In addition, it is also a common practice to provide a portable space heater with a high temperature limiting device, usually in the form of a non-adjustable, or else a factory-adjustable, thermostat. An important purpose of a high temperature limiting device is to avoid the risk of fire due to certain abnormal conditions in which a heater may be used. An overheated portable heater may also cause personal injury from excessive heat or electric shock.
One cause of overheating is the placement of the heater heating elements so close to another object that the object is excessively heated. This could occur, for example, when a heater which is to be placed with its back close to a wall is instead placed with its front close to the wall. It could also occur when a heater is tipped over on its front face. Another problem which is considered to be an overheating problem is the risk of fire created when a piece of flammable fabric or other flammable material is draped over the front of a heater.
Many of the circumstances which can cause overheating can be handled by temperature sensors placed in essentially any location in front of the heating elements. However, some causes of overheating require more sophisticated approaches. For example, Underwriter's Laboratories, Inc. (UL) has a requirement that portable space heaters must meet certain so-called "drape" test criteria in which a piece of fabric material draped over only parts of the front of the heater will not catch on fire. To meet some drape tests, it is necessary to provide a high temperature limiting detector or detectors capable of detecting localized hot spots that may be created at different locations in the front of the heater. Plural overheat detectors have been used in an effort to meet these tests, but the use of plural detectors does not provide assurance against the formation of localized hot spots.
Therefore, there is a need for an improved heater which can provide adequate protection against overheating in the event it is operated under certain abnormal conditions, particularly when the heating element or elements are too close to other objects, and particularly when the other objects are too close to only a portion of the front of the heater so that any overheating is localized.
Capillary tube thermostats have been used for many years in baseboard heaters because they are capable of detecting localized hot spots over substantially the entire lengths of sections of the baseboard heaters. However, in so far as known, capillary tube thermostats have not been used in radiant heaters since they are not readily heated by radiant heat energy.
A problem experienced with conventional radiant electric space heaters is that their heating chambers, being open to the outside of the heater, often get quite dirty after extended use. Accordingly, it would be desireable to enable one to clean the heating chamber. However, the grills used to cover the windows to the heating chambers, the presence of which prevents throrough cleaning of the heating chambers, are, for purposes of safety, normally difficult or impossible to remove.